In image forming apparatuses such as a copying machine unit, a printer unit, a facsimile machine unit, a printing machine unit, a multi-functional unit having functions of previously listed units, etc., visible images such as a toner image, etc., which is borne on a lateral image bearing body are transferred to a sheet-shaped recording medium (below called a sheet) to obtain an image output. The toner image is fixed onto the recording medium with fusion and penetration when it passes through a fixing apparatus. In this way, heating schemes adopted for the fixing apparatus include a thermal roller fixing scheme which is provided with a heating roller such as a halogen lamp, etc., as a heat generating source and a pressurizing roller which opposes and abuts against the heating roller to form a fixing nip portion; a film fixing scheme which uses, as a heating member, a film which requires a smaller heat capacity than a roller itself, etc. Recently, fixing schemes which use electromagnetic induction heating for the heating scheme (see Patent document 1, for example) have been attracting attention.
The electromagnetic induction heating scheme is provided with a configuration such that an induction heating coil wound onto a bobbin inside the heating roller is provided and an electric current is applied to the induction heating coil to generate an eddy current to thereby generate heat in the heating roller, so that preheating such as in the thermal roller fixing scheme which uses the halogen lamp, etc., is not needed, making it possible to raise the heating roller to a predetermined temperature near instantaneously.
Moreover, with respect to the electromagnetic induction heating scheme, a fixing apparatus is known which includes a high frequency induction heating apparatus including an induction heating coil to which a high frequency voltage is applied with a high frequency power supply, and a magnetic heat-generating layer provided at a heating rotor such as the above-described heating roller, wherein a material that has a Curie point which is generally a fixing temperature is used to form the heat generating layer, and the high frequency voltage is applied to the high frequency induction heating apparatus by the high frequency power supply to obtain heat generation necessary for fixing (see Patent document 2, for example).
The apparatus disclosed in Patent document 2 includes an adhesive layer in which a ferromagnetic material is dispersed on a surface of a core bar in the high frequency induction heating apparatus. The adhesive layer instantaneously rises in temperature until the ferromagnetic material contained in the adhesive reaches the Curie temperature, and loses magnetism when the Curie temperature is reached, so that the temperature does not continue to rise, thereby maintaining a constant temperature. The Curie temperature of the above-mentioned ferromagnetic material is generally set at the fixing temperature, so that the ferromagnetic material is generally maintained at the fixing temperature. Therefore, a rise time of a heating rotor may be shortened and highly accurate temperature control may be performed without undermining high releasability, heat resistance, etc., of a surface of the heating rotor that are properties required of a fixing apparatus and also without needing a complex control apparatus.
In such a fixing apparatus which performs self control of an induction heating amount using a magnetic shunt alloy, a scheme is adopted such that a magnetic shunt layer made of the magnetic shunt alloy is disposed between an induction coil and a degaussing member, and when the magnetic shunt alloy reaches the Curie temperature or above, a repulsive magnetic flux due to the degaussing member cancels out an induction magnetic flux to demonstrate a self temperature control function. With reference to portions of Patent document 3 by the present applicant that includes the features which are common to the present invention, one example thereof is generally described as a reference example to clarify the problems.
(Reference Example)
In a fixing apparatus shown in FIG. 12, a fixing roller 3, which opposes and abuts against a pressurizing roller 4, rotates in an arrow direction. In the vicinity of an outer periphery of the fixing roller 3, a magnetic flux generator 2 is fixed to the fixing apparatus body (not shown).
The magnetic flux generator 2 includes a center core 2c at a center; an arch core 2d, which includes leg cores 2b, etc., on both ends; an exciting coil 2a, etc. The exciting coil 2a, which is disposed between the arch core 2d and the fixing roller 3, is a flat coil wound around the center core 2c as also shown in FIGS. 13 and 14.
In FIG. 14, an inverter E, which is a drive source, drives the exciting coil 2a with a high frequency to generate a high frequency magnetic field (magnetic flux), which causes an eddy current to flow in a fixing sleeve 3H of primarily metal that forms an outer peripheral portion of the fixing roller 3, so as to increase the temperature of the roller 3. A sheet S bearing toner Tn undergoes heating, pressurizing, and fixing when it passes between the fixing sleeve 3H and the pressurizing roller 4 such that a toner face contacts the fixing sleeve 3H.
FIG. 15 shows, in a schematic visual cross-section cut in a radial direction, a portion of the fixing roller 3 that includes, on the innermost side, a degaussing member 5 which also serves as a cored bar, an insulating layer 3B formed of air (or foam), a magnetic shunt layer 3C, an antioxidant layer 3D1, a heat generating layer 3E, an antioxidant layer 3D2, an elastic layer 3F, and a releasing layer 3G. The magnetic shunt layer 3C, the antioxidant layer 3D1, the heat generating layer 3E, the antioxidant layer 3D2, the elastic layer 3F, the releasing layer 3G, etc., make up the fixing sleeve 3H, which is an integral heat generating rotor.
The degaussing member 5 in FIG. 15 is made of aluminum or an aluminum alloy, while the insulating layer 3B formed of air is a gap of about 5 mm, for example. The magnetic shunt layer 3C is made of an appropriate magnetic shunt alloy having a thickness of 50 μm, for example. The antioxidant layers 3D1 and 3D2 are nickel strike plated and have a thickness of less than or equal to 1 μm, for example. The heat generating layer 3E is Cu plated and has a thickness of 15 μm, for example. The elastic layer 3F is made of a silicone rubber and has a thickness of 150 μm, for example. The releasing layer 3G is made of PFA having a thickness of 30 μm, for example. A thickness from the magnetic shunt layer 3C to the surface of the releasing layer 3G is approximately 200-250 μm, for example.
The magnetic shunt layer 3C includes a magnetic material (e.g., a magnetic shunt alloy material including iron and nickel) which is formed such that the Curie temperature falls in a range of 100-300° C., for example, and is always disposed between the exciting coil 2A and the degaussing member 5. The magnetic shunt layer 3C prevents the heat-generating layer 3E, etc., from being overheated. The degaussing member 5 is a cylindrically shaped roller and has a circular shape concentric with the fixing sleeve 3H.
With reference to FIGS. 16A and 16B, a heat generation preventing function by the degaussing member 5 in “a self temperature control-type fixing apparatus using a magnetic shunt alloy” is described.
(1) FIG. 16A shows a non-functioning state, which is without the heat generation preventing function, as the magnetic shunt layer 3C is at a temperature lower than the Curie temperature. As shown, a temperature T of the magnetic shunt alloy which makes up the magnetic shunt layer 3C is lower than a Curie temperature Tc (T<Tc). A bold solid line arrow shows induction magnetic flux from the exciting coil 2a, while a thin solid line arrow shows an eddy current flowing in the magnetic shunt alloy. As the temperature T of the magnetic shunt alloy is lower than the Curie temperature, the magnetic shunt alloy within the fixing sleeve 3H remains as a magnetic material, and the induction magnetic flux which the exciting coil 2a generates does not penetrate through the magnetic shunt layer 3c. 
As the magnetic shunt alloy has magnetism at the above-described temperature of lower than the Curie temperature, it does not pass through the induction magnetic flux from the exciting coil 2a in an arrangement such that the magnetic shunt alloy is disposed between the exciting coil 2a and the degaussing member 5, so that the induction magnetic flux does not reach the degaussing member 5, and a repulsive magnetic field is not generated in the degaussing member 5, so that there is no suppressing of heat generation by the magnetic shunt alloy. Therefore, the heat generating layer 3E generates heat due to the induction magnetic flux of the exciting coil 2a, which heat is transferred to and sensed by the magnetic shunt layer 3C, making it possible to rapidly increase its temperature to a temperature near the Curie temperature.
(2) FIG. 16B shows a functioning state, which is with the heat generation preventing function, as the magnetic shunt layer 3C is at a temperature exceeding the Curie temperature. As shown, the temperature T of the magnetic shunt layer 3C exceeds the Curie temperature Tc and loses magnetism, so that, in an arrangement such that the magnetic shunt alloy is disposed between the exciting coil 2a and the degaussing member 5, the induction magnetic flux (shown in a bold line) from the exciting coil 2a penetrates through the magnetic shunt layer 3C and the insulating layer 3B to reach the degaussing member 5. Thus, the induction magnetic flux from the exciting coil 2a passes through the degaussing member 5. When the time-varying induction magnetic flux penetrates the degaussing member 5 (conductor), an induction current (an eddy current shown as a thin solid line) flows in the degaussing member 5, the induced eddy current acts toward canceling out the induction magnetic flux, and, in conjunction therewith, a repulsive magnetic flux (shown in a dotted line) which cancels out the induction magnetic flux is induced. The repulsive magnetic flux cancels out the induction magnetic flux from the exciting coil 2a, so that heat generating efficiency of the heat generating layer 3E due to the induction magnetic flux from the exciting coil 2a decreases.
As shown in FIG. 16A, the magnetic shunt layer 3C, which is a magnetic body and which includes a heat generating layer, almost instantaneously rises in temperature until it reaches the Curie temperature. However, as shown in FIG. 16B, when the Curie temperature is reached (i.e., when T>Tc), magnetism is lost, so that a temperature rise due to induction heating does not occur, maintaining a constant temperature. Thus, this is a self temperature control function which utilizes the Curie temperature by mutual interaction of the exciting coil 2a, the degaussing member (cored bar) 5, the magnetic shunt layer 3C, and the heat generating layer 3E.
Therefore, when the magnetic shunt alloy 3C is arranged such that the Curie temperature of a material which makes up the magnetic shunt alloy 3C falls in a range of 100-300° C., which is a range of temperature used in this type of fixing apparatus, the degaussing member 5 and the heat generating layer 3E of the fixing sleeve 3 may not overheat, so that a fixing temperature may generally be maintained, without undermining high releasability and heat resistance at the surface of the fixing sleeve 3H and without needing complex control.
(Problems with the Above-Described Reference Example)
FIG. 17 is a diagram showing temperature dependence of permeability (heat generation efficiency) of the magnetic shunt layer 3C. As shown, Δ denotes the permeability at the respective temperatures. In the fixing apparatus of the present example, a self temperature control function acts, so that it is easy to perform temperature control at a setting temperature near 180° C. (a fixing temperature which is set near the Curie temperature). However, as seen from FIGS. 16A and 16B, the permeability is very high at less than the setting temperature (a fixing temperature which is set near the Curie temperature), but drastically decreases beyond the setting temperature. Therefore, as the magnetic shunt layer 3C greatly falls in permeability near the Curie temperature, magnetic flux penetrates through the degaussing member 5, and the self temperature control function is demonstrated due to a repulsive magnetic flux from the degaussing member 5, so that it is difficult to heat the apparatus to a temperature which is higher than or equal to the Curie temperature.
In this way, in a self temperature control type fixing apparatus which uses a magnetic shunt alloy, there is a problem that it is not possible to rapidly perform warming up since heat generating efficiency decreases as the temperature of the heat generating body approaches the Curie temperature. As a countermeasure, it is possible to increase the Curie temperature of the magnetic shunt alloy such that heating to a high temperature can be performed. However, in this case, as an upper-limit temperature for a temperature of an end portion at the time of paper passing rises, a problem occurs that a difference in luster becomes large between a small-sized paper passing section and non-paper passing section as shown in FIG. 18, for a large-sized (for example, an A3 paper) image immediately after consecutive small-sized papers passing, etc.
Thus, as related art, in Patent document 3 is proposed, as in the following, a fixing apparatus which heats with an exciting magnetic flux from an induction coil, wherein a magnetic shunt alloy layer is disposed between the induction coil and a degaussing member, and which enables control in a manner such that, when a self temperature control property is to be demonstrated, the degaussing member demonstrates a degaussing function to generate a magnetic flux repulsing an induction magnetic flux, and, when a self temperature control function is not demonstrated, the degaussing member does not demonstrate the degaussing function, and, at the timing of warming up, the self temperature control function is not demonstrated, so that a high speed launching is implemented.
FIGS. 19A and 19B show configurations and operating states of the magnetic flux generator 2 and a fixing roller 30 where the configuration of the magnetic flux generator is the same as the above-described FIGS. 12 and 13. For the fixing roller 30, the fixing sleeve 3H has the same basic structure as that shown in FIG. 15. The feature of the fixing roller 30 of the present example differs from the feature of the fixing roller 3 in that, as the degaussing member 5 shown in FIGS. 12 and 15, a pair of degaussing coils 3L as a degaussing member is provided inside the fixing sleeve 3H which includes the magnetic shunt layer 3C. The degaussing coil 3L is supported such that it holds an arrangement which opposes the exciting coil 2a with the fixing sleeve 3H inbetween.
In FIG. 20 is shown the manner in which the degaussing coil 3L is supported. Side plates 8L and 8R of the fixing apparatus support the magnetic flux generator 2 and support the fixing roller 30 via bearings. The fixing sleeve 3H is a heat generating rotor which forms an outer peripheral portion of the fixing roller and is fixed to flanges 7R and 7L. An internal member 66, which is disposed inside (on the inner side) of the fixing sleeve 3H and which immovably supports the degaussing coil 3L, etc., so as to be non-rotating relative to the rotating fixing sleeve, has its right axle 6R supported by the flange 7R via a bearing 6. An axle 9R of the flange 7R that penetrates the side plate 8R is supported via a bearing and connected to a rotational drive source (not shown). A left axle 6L of the internal member 66 is supported by the flange 7L via the bearing 6 and penetrates the flange 7L to protrude toward outside, and an axle 9L of the left flange 7L is fixed to the left side plate 8L of the fixing apparatus. In this way, the fixing sleeve 3H is arranged to rotate between the stationary degaussing coils 3L, 3L and the stationary magnetic flux generator 2.
With reference to FIG. 21, a switching element 16 is turned on to short the degaussing coil 3L (to make the degaussing coil 3L conductive), or is turned off to make the degaussing coil 3L non-degaussing, so that it has a switching function which is designed to suppress the induction magnetic flux by the exciting coil 2a. While a relay switch, a semiconductor switch, a variable resistive element, etc., may be used as the switching element 16, other means may be used. Moreover, the degaussing coil 3L is not provided with a drive source. Moreover, as shown in FIGS. 19A and 19B, relative to the exciting coil 2a which is divided into two halves with a center core 2c inbetween, one degaussing coil 3L is arranged for each side on the axial longitudinal direction with an air gap at a center portion on the axial longitudinal direction of the fixing roller relative to the exciting coil 2a. Preferably, multiple degaussing coils 3L, about 3, for example, may be provided. In the present invention, one or more of the degaussing coils 3L may be provided, so that, there is no limit to the number. Then, control is performed according to a rate of switching by the switching element 16 per unit time.
(Degaussing Coil in Conducting State: with Heat Generation Suppressing Function)
FIG. 19A illustrates a cross section of the fixing sleeve 30, showing an operation state for enhancing the degaussing function. The fixing sleeve 3H which includes the magnetic shunt layer 3C is disposed between the exciting coil 2a and the degaussing coil 3L. Moreover, the degaussing coil 3L as an example of a degaussing member is arranged at a position such that the induction magnetic flux (solid line) of the exciting coil 2a passes through the magnetic shunt layer 3c to reach the degaussing coil 3L.
At T>Tc, the switching element 16 is turned on to short the degaussing coil 3L (to make the degaussing coil 3L conductive). In this way, a current is induced in the degaussing coil 3L in a direction for canceling out the induction magnetic flux from the exciting coil 2a, causing a repulsive magnetic flux (degaussing magnetic flux) shown with a broken line arrow to be generated to cancel out and weaken the induction magnetic flux from the exciting coil 2a. The switching element 16 is switched on to suppress heat generation of the heat generating layer 3E.
When the magnetic shunt layer 3C is at a temperature which is higher than or equal to the Curie temperature, the induction magnetic flux (solid line) from the exciting coil 2a may pass through. When the temperature of the magnetic shunt layer 3 is near the Curie temperature or at a temperature which is higher than but close to the Curie temperature, as the repulsive magnetic flux from the degaussing coil 3L increases, the induction magnetic flux due to the exciting coil 2a decreases, so that an eddy current due to the induction magnetic flux at the heat generating layer 3E and the amount of heat generated decreases.
When the amount of heat generated decreases, the temperature of the magnetic shunt layer 3C decreases to the Curie temperature and in conjunction the magnetic flux which passes through the magnetic shunt layer 3C decreases and the repulsive magnetic flux decreases. However, the induction magnetic flux which passes through the heat generating layer 3e increases in correspondence with the decreased repulsive magnetic flux, so that an amount of heat generated increases. In this way, the amount of heat generated at the heat generating layer 3E is automatically controlled such that the magnetic shunt layer 3C reaches a temperature near the Curie temperature. The above-described state corresponds to a characteristic line which connects the Δ symbols at a setting temperature of 200° C. or higher in FIG. 22.
Here, as shown in FIG. 19A, assuming a degaussing member functioning state (an ON state of the switching element 16), the induction magnetic flux of the exciting coil 2a cannot pass through the magnetic shunt layer 3C, so that the repulsive magnetic flux due to the degaussing coil 3L is not generated. Thus, the induction magnetic flux due to the exciting coil 2a makes it possible to generate an eddy current at the heat generating layer 3E without any constraint and to cause heat to be generated in the heat generating layer 3E as much as possible. The above-described state corresponds to a characteristic line (maximum amount of heat generated of 1000 W) which connects the Δ symbols at the setting temperature of 180° C. or below in FIG. 22.
(Degaussing Coil in Non-Conducting State: without Heat Generation Suppressing Function)
On the other hand, FIG. 19B is a cross-sectional diagram of the fixing roller 30 that shows an operation state in which the degaussing function is not demonstrated. The switching element 16 is turned off to block the degaussing coil 3L and cause the degaussing magnetic flux to not be generated, so that the degaussing function is not demonstrated.
The degaussing coil 3L is disposed away from and on the opposing side of the exciting coil 2a with the fixing sleeve 3H inbetween. When the temperature T of the magnetic shunt alloy is higher than the Curie temperature Tc, the induction magnetic flux from the exciting coil 2a penetrates through the magnetic shunt layer 3C, but the degaussing coil 3L is blocked, so that the induction repulsive magnetic flux is not generated. Thus, the induction magnetic flux (solid line) generates an eddy current in the heat generating layer 3E without constraint and causes heat to be generated in the heat generating layer 3E. The above-described state corresponds to a characteristic line (maximum heat value of 1000 W) which connects the ◯ symbols at the setting temperature of 180° C. or higher in FIG. 22.
Assuming a non-degaussing functioning state in which the degaussing coil 3L of FIG. 19B has the switching element 16 turned off, even in this case, in the heat generating layer, an eddy current is generated without constraint, causing heat to be generated. The above-described state, which corresponds to a characteristic line (maximum heat value of 1000 W) which connects the ◯ symbols at the setting temperature of 180° C. or less in FIG. 22, makes it possible to perform heat generation of the heat generating layer as much as possible.
In this way, the switching element 16 constitutes one example of a magnetic flux adjusting unit in that the switching element 16 acts on the degaussing coil 3L in operation modes of on and off in which a circuit which includes the degaussing coil 3L is turned on and off to adjust the repulsive magnetic flux.
While omitted in FIGS. 19A and 19B for convenience of explanations, in practice, as shown in FIGS. 23A and 23B, a nip member 55 which is pressurized by a pressurizing roller 4 is immovably provided at a portion which is inside the fixing sleeve 3H and which opposes the pressurizing roller 4 and is supported by the immovable internal member 66. Here, immovable means immovable relative to the rotating fixing sleeve 3H.
Here, what may be a problem is that there is the internal member 66. As shown in FIGS. 23A and 23B, the internal member 66 immovably supports the nip member 55 inside the rotating fixing sleeve 30. Moreover, the internal member 66 may support the degaussing coil 3L which similarly needs to be immovably supported as well as other members which need to be immovably supported.
FIG. 23A, which corresponds to FIG. 19B, additionally includes the internal member 66 and the nip member 55 therein in order to describe the impact of the internal member 66 when the switching element 16 is off. In FIG. 23A, where T<Tc, the temperature T of the magnetic shunt layer 3C which is included in the fixing sleeve 3H is lower than the Curie temperature, so that, when the degaussing coil 3L is turned off and in the non-degaussing functioning state, the induction magnetic flux due to the exciting coil 2a may not pass through the magnetic shunt layer 3C, so that the repulsive magnetic flux due to the degaussing coil 3L is not generated. Thus, the induction magnetic flux due to the exciting coil 2a makes it possible to generate an eddy current in the heat generating layer 3E without any constraint, there is no heat generation suppressing, leading to a normal heat generating state, so that there is no particular problem.
However, when the temperature T of the magnetic shunt alloy which forms the magnetic shunt layer 3C under the condition in FIG. 23A rises to a temperature near the Curie temperature, the induction magnetic flux due to the exciting coil 2a passes through the magnetic shunt layer 3C as shown in FIG. 23B. However, the switching element 16 is in an off state, so that the repulsive magnetic flux by the degaussing coil 3L is not generated, but the induction magnetic flux due to the exciting coil 2a penetrates through the degaussing coil 3L, so there is a problem that the magnetic flux, which has penetrated through the degaussing coil 3L, is induced by the internal member 66 which includes an induction heating body which supports the degaussing coil 31, the nip member 55, etc., so that an eddy current is generated, causing a heat generating loss 80 to be generated.